This invention relates to low pressure, chemical vapor deposition (LPCVD) of films and, in particular, of tungsten films.
In silicon semiconductor devices, contact is made to a device by a deposited metal conductor. The contact area of silicon is usually at the bottom of an opening or via. Thus, one must provide a continuous conductor over the edge of the via and down the sidewall, so-called "coverage", well as between devices.
As one produces more and more devices in a given area of silicon, all geometries must shrink, including the size of the contact area. The thicknesses of overlying layers of dielectric or conductor do not shrink, at least not proportionately. The result is a via with a smaller width to height ratio. This worsens the step coverage problem.
One solution is to selectively fill the vias with tungsten by way of an LPCVD process to provide a relatively planar surface to cover with conductor. This solution is not without difficulty since the deposition process causes the formation of volatile silicon compounds, i.e. the silicon is etched. This can cause excessive junction leakage or even shorting. In addition, the geometry of the via is altered (widened) as the silicon is consumed. The latter, etching in a direction parallel to the surface of the wafer, is referred to as lateral encroachment. The former, etching in a direction perpendicular to the surface of the wafer, is referred to as silicon consumption.
Tungsten is typically deposited at approximately 300.degree. C. It has been reported in the literature that tungsten deposition by silicon reduction (3Si+2WF.sub.6 .fwdarw.2W+3SiF.sub.4) is self-limiting at a thickness of about 20 nm. It is further reported that additional tungsten can be deposited without additional silicon consumption by using hydrogen reduction (3H.sub.2 +WF.sub.6 .fwdarw.W+3HF), which is not self-limiting. At low temperatures, the deposition rate is too slow for filling deep vias, e.g. one micron or deeper. Thus, it is desirable to deposit tungsten at higher temperature, e.g. 500.degree. C., with high partial pressure of tungsten hexafluoride, e.g. 10 mtorr, in the presence of hydrogen to achieve high growth rate. However, tungsten deposited under these conditions shows unpredictable and nonreproducible results.
In view of the foregoing, it is therefore an object of the present invention to provide an improved process for the deposition of tungsten.
Another object of the present invention is to reduce lateral encroachment in the deposition of tungsten on silicon.
A further object of the present invention is to reduce silicon consumption in the deposition of tungsten on silicon.
The foregoing objects are achieved in the present invention wherein the wafer is subjected to a low temperature deposition of tungsten by hydrogen reduction of tungsten hexafluoride. A tungsten layer approximately 200 .ANG. thick is deposited: enough to assure complete coverage of the bottom of the via. The layer is then subjected to an anneal in a nitrogen atmosphere at a higher temperature than the deposition. The balance of the tungsten layer is then deposited, e.g. also by hydrogen reduction of tungsten hexafluoride.